The present invention generally relates to a method and apparatus for cleaning contaminant material from structural surfaces. More particularly, the present invention relates to cleaning residual contaminant material from the surfaces of a structure by directly or indirectly utilizing a cryogenic or refrigerant liquid.
Cryogenics is the branch of science dealing with properties and effects of materials at very low temperatures. Cryogenic technology has found widespread industrial use in such applications as surgery, life-support systems for outer space environments, vacuum systems, superconductive devices, preservation of food and certain biological matter, and as missile propellant systems, to name a few. Generally, cryogenic technology utilizes gaseous substances which have been liquified by maintaining the gases under constant increased pressure above the triple point of the gases. The triple point of a substance is that pressure and temperature at which three different phases of one substance can co-exist in equilibrium.
Bulk transporation or storage of liquid or viscous materials, such as, for example, petroleum products, polymer resins, organic chemicals or the like, pose certain problems for the operator of transporation or storage facilities. In particular, it is necessary for the operators of the bulk transportation equipment to be able to off-load the cargo quickly and to efficiently load new cargo such that idle time during the off-loading and on-loading operations is minimized. The equipment utilized by these facilities will, thus, be maintained in commerce and will be productive by minimizing the idle time.
However, a transporter of bulk materials as described above has particular problems when it is desired to off-load one type or quality of material and subsequently on-load a material of different type or quality. For example, it may be desirous to off-load a low quality petroleum product, such as coal tar and, subsequently, load a higher grade of petroleum product or the like. Therefore, it is extremely important that the transporting equipment be cleaned as thoroughly as possible so that the contamination of the newly on-loaded material is minimized.
It has been proposed that certain cleaning methods and techniques be utilized to clean transport equipment. For example, cleaning solvents and chemical cleaning solutions have been proposed, such as, for example, those disclosed in U.S. Pat. Nos. 3,436,263, 3,536,529, and 3,914,132. It has also been proposed to contact and remove contaminating material by utilizing water and/or steam as disclosed in U.S. Pat. Nos. 3,182,669 and 3,746,023.
A distinct problem associated with utilizing the chemical solvent technique of cleaning surfaces, and especially those surfaces in a cargo hold or the like in bulk transport equipment, is that certain of the chemicals may be expensive or could be hazardous to the person operating or performing the cleaning operation. Additionally, the chemical solvent or water techniques for cleaning surfaces in a cargo hold may not be adaptable for certain chemical or viscous materials, such as, for example, polyresins, inorganic resins, petroleum products and by-products or the like.
Cryogenic liquids, such as liquid carbon dioxide (CO.sub.2) and liquid nitrogen (N.sub.2) have been proposed to be sprayed upon environmental oil spills in order to mitigate the environmental impact and damage from such oil spills. Such methods and apparatus for applying the liquid cryogen to the uncontrolled oil spills have been disclosed in U.S. Pat. Nos. 3,614,873, 4,043,140 and 4,157,016.
Generally, the cryogenic cleaning methods applied to environmental oil spills utilize an apparatus which applies the cryogenic material and subsequently removes a combination of frozen oil and substrate upon which the oil is associated. For example, in U.S. Pat. No. 4,043,140 a method and apparatus are disclosed whereby an oil spill on a beach is frozen when sprayed with liquid cryogen. Subsequently, the apparatus removes the combination of frozen oil and beach sand and separates the two components. In U.S. Pat. No. 3,614,873, a similar apparatus is disclosed which sprays cryogen material on an oil spill occurring on a body of water. The cryogen freezes the oil floating on the surface of the water and is subsequently skimmed from the water surface by the apparatus disclosed.